That's a typical modern lead at what appears to be 45 degrees. I have two in .40-50SS I shoot, and one is a modern GM barreled Rolling Block, the other a Schoyen barreled forged Ballard. The Schoyen has a long lead of around 11 degrees, but I shoot both using my RCBS CSA 370 gr. cast bullet. 
There's no reason to need more case volume if you load it with smokeless, but might need more if you shoot it with BP. I use 4198 powder and there's plenty of space for a good load. I think if smokeless loads had been around before the .40-50SS became out of production it might have stayed around a lot longer. It really shines with smokeless in the short case.
I have a bunch of rifles in .40-70 and love them, but the .40-50 with smokeless is just as much fun, and the smaller case makes it more efficient.

Ronnie, with the 18 twist a 400 grain might not hold up at distance. I tried that with my 40-60 Maynard and it didn't. Consulting with Dan Theodore he said with an 18 twist better try the 370 grain Saeco and it got me 30 animals first time out. How much BP compressed could you get in your brass with a 370 grain seated as far out as you can? You may not need much in that smaller case.

With BP you're better off shooting a 350-370 grain bullet and maybe even try some compression to get more velocity in the .40-50SS. Velocity will make up for the 18 twist, but BP is going to limit case capacity just like it did with the original .40-50SS cartridges. I shoot my Ballard in .40-85 Everlasting using a 410 grain Snover bullet and it's a 21" twist rate. It shoots very well with the heavy bullet, but I'm using 4198 at around 1350 fps. Not sure you'll get there with BP?

I wouldn't rechamber it to the .40-70SS unless you've got a good supply of brass already. That brass, and .405 Hornady brass are all tough to find, and expensive also. The donor brass for .40-50SS is about as cheap as brass could get these days! Might consider a wildcat using .30-40 Krag or .303 British brass expanded to .40 caliber, but left full length. It will be shorter than .40-70SS, but much longer than .40-50SS and easy to load for using .40-70 dies.

Ronnie, I don't compress my Swiss. I use 53 grains 1.5 and a 400 grainer in my 16 twist 40-65 and in the 18 twist 40-60 was using 50-55 grains with the 370. Yes, it is the 640 and you should be able to seat it out to the rifling to get more room in the case and if necessary compress some to get at least 50 grains in. Try a case and see how much powder you can get in to leave 1/4 inch for seating the bullet.